The invention relates to an ink-jet system comprising an ink channel between an ink reservoir and a nozzle, and an electromechanical transducer which comprises an expansible member arranged adjacent to the ink channel for abruptly reducing the volume of the same in order to eject an ink droplet through the nozzle. Such ink-jet systems are used as printheads in ink-jet printers.
A drop-on-demand ink-jet system of the type indicated above is known, for example, from EP-B1-0 402 172. In this known system, the ink channel is formed in a substrate which is sandwiched between a bottom plate and a cover plate such that the top and bottom surfaces of the ink channel are formed by the cover plate and the bottom plate, respectively. The ink channel has a constant depth which is identical to the height of the nozzle, but has a larger width than the nozzle and is tapered at its front end so that its width is gradually reduced to that of the nozzle. The expansible member of the electromechanical transducer is formed by a plate-like piezoelectric element which is disposed underneath the bottom plate within the area of the ink channel. The piezoelectric element is supported on a rigid support plate and has its top end face directly engaged with the bottom plate of the ink channel. When an electric voltage is applied to the piezoelectric element, the piezoelectric material expands in the vertical direction, and the elastic bottom plate is flexed inwardly of the ink channel, so that an ink droplet is expelled from the nozzle.
U.S. Pat. No. 5,119,116 discloses a thermal ink-jet system in which the ink channel is provided with a step structure such that the height of the nozzle is smaller than the depth of the main portion of the ink channel. The pressure required for expelling an ink droplet from the nozzle is formed by a bubble-generating heating element disposed in a pit which is formed in the bottom of the ink channel upstream of the step structure.
In a practical printhead for high-speed and high-resolution printing, a plurality of ink-jet systems are integrated on a common substrate. In order to achieve objectives like large-scale integration, a high maximum frequency of drop generation and the like, the ink-jet systems should be made as compact as possible. On the other hand, the ink-jet systems should be operable with moderate voltages and must nevertheless be capable of providing a sufficient energy for creating droplets of a suitable size and accelerating them to a suitable speed so that the droplets may be deposited on the recording medium with high accuracy.
It is therefore an object of the invention to improve the energy efficiency of the ink-jet system. According to the invention, this object is achieved with an ink-jet system in which the depth of a portion of the ink channel between the expansible member and the nozzle is larger than both the depth of the portion adjacent to the expansible member and the height of the nozzle.
It has been found that this construction provides a significant improvement in the efficiency with which the electric energy applied to the transducer is converted into kinetic energy of the droplet.
The total energy efficiency depends largely on the following two factors: (1) The efficiency with which the electric energy of the transducer is converted into energy of an acoustic wave propagating in the ink liquid and (2) the efficiency with which the acoustic energy is conferred to the droplet created at the nozzle.
The first factor is determined by the ratio between the depth of the ink channel and the thickness of the expansible member of the transducer, e.g. the piezoelectric element. Ideally, this ratio should be substantially equal to the ratio between the elastic modulus of the piezoelectric material and the ink liquid. Since the piezoelectric material generally has a comparatively large elastic module and, on the other hand, the thickness of this element is limited by practical constraints, this factor requires a rather small depth of the ink channel.
The second factor depends on the ratio between the sectional areas of the nozzle and the ink channel. Ideally, this ratio should be so selected that an optimal xe2x80x9cimpedance matchxe2x80x9d is provided for the acoustic wave, in order to avoid energy losses by reflection of the acoustic wave. Since the cross-section of the nozzle is determined by the desired size of the droplets and the width of the ink channel should not be made too large, a comparatively large depth of the ink channel would be desirable in view of this factor.
According to the invention, both factors are brought closer to the optimum by. selecting a rather small depth for the portion of the ink channel adjacent to the transducer and by increasing the depth of the portion of the channel adjacent to the nozzle in order to achieve a better impedance match. Computer simulations have shown that, in this way, the total energy efficiency can be increased in the order of a factor of ten.
In one embodiment, the depth of the portion of the ink channel between the transducer and the nozzle is gradually increased from the transducer towards the nozzle. Since, in this case, there are only smooth transitions in the depth of the channel upstream of the nozzle, energy losses due to reflections of the acoustic wave can be reduced.
It has been found however that it is not always necessary to avoid reflective structures in the ink channel upstream of the nozzle and that, in fact, such reflective structures may even be beneficial in terms of energy efficiency.
In another embodiment of the invention, the portion of the ink channel between the transducer and the nozzle is therefore designed as a cavity which causes partial reflection of acoustic waves at both the upstream and downstream ends thereof. In this case the cavity can serve as an energy accumulator which can trap or accumulate acoustic energy in order to provide maximum power at the moment at which a droplet is to be generated.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.